docs: def savepysyn(self,wave,flux,fname,units=None):
          """ Cannot ever use the .writefits() method, because the array is
          frequently just sampled at the synphot waveset; plus, writefits
          is smart and does things like tapering."""
          if units is None:
              ytype='throughput'
              units=' '
          else:
              ytype='flux'
          col1=pyfits.Column(name='wavelength',format='D',array=wave)
          col2=pyfits.Column(name=ytype,format='D',array=flux)
          tbhdu=pyfits.BinTableHDU.from_columns(pyfits.ColDefs([col1,col2]))
          tbhdu.header.update('tunit1','angstrom')
          tbhdu.header.update('tunit2',units)
          tbhdu.writeto(fname.replace('.fits','_pysyn.fits'))